Zero Energy Windows

Windows in the U.S. consume 30 percent of building heating and cooling energy, representing an annual impact of 4.1 quadrillion BTU (quads) of primary energy. Windows have an even larger impact on peak energy demand and on occupant comfort. An additional 1 quad of lighting energy could be saved if buildings employed effective daylighting strategies.

The ENERGY STAR(r) program has made standard windows significantly more efficient. However, even if all windows in the stock were replaced with today's efficient products, window energy consumption would still be approximately 2 quads. However, windows can be "net energy gainers" or "zero-energy" products. Highly insulating products in heating applications can admit more useful solar gain than the conductive energy lost through them. Dynamic glazings can modulate solar gains to minimize cooling energy needs and, in commercial buildings, allow daylighting to offset lighting requirements. The needed solutions vary with building type and climate. Developing this next generation of zero-energy windows will provide products for both existing buildings undergoing window replacements and products which are expected to be contributors to zero-energy buildings.

This paper defines the requirements for zero-energy windows. The technical potentials in terms of national energy savings and the research and development (R&D) status of the following technologies are presented:

• Highly insulating systems with U-factors of 0.1 Btu/hr-ft²-°F
• Dynamic windows: glazings that modulate transmittance (i.e., change from clear to tinted and/or reflective) in response to climate conditions
• Integrated facades for commercial buildings to control/ redirect daylight

Market transformation policies to promote these technologies as they emerge into the marketplace are then described.